Adaptive tracking of geological objects

ABSTRACT

The present invention provides a computer-implemented method for detecting at least one natural contour of a geologic object in 3D seismic data, the method comprising the steps of: (a) receiving at least one first predetermined data set from said 3D seismic data comprising a plurality of phase events; (b) selecting at least one first seed phase event having a first phase characteristic from said plurality of phase events; (c) determine a characterising score between said selected at least one first seed phase event and each one of a predetermined number of candidate phase events of said at least one first predetermined data set; (d) assign said characterising score to each one of said predetermined number of candidate phase events; (e) adjust said characterising score of at least one of said predetermined number of candidate phase events in accordance with at least one first boundary condition; (f) determine at least one natural contour between said at least one first seed phase event and at least a second phase event, utilising an optimisation algorithm; (g) generate a visual representation of said at least one natural contour within said at least one first predetermined data set.

The present invention relates generally to the field of oil and gasexploration, and in particular to the field of computer aidedexploration for hydrocarbons using geophysical data, such as for exampleseismic data, of the earth. Even more particular, the present inventionrelates to the analysis of seismic trace data, so as to allowinteractive tracking of geological objects, such as horizons and faults.

INTRODUCTION

In the oil and gas industry, geological data surveys such as, forexample, seismic prospecting and other similar techniques are commonlyused to aid in the search for and evaluation of subterranean hydrocarbondeposits. As an example, prospecting operations include three basicstages (i) data acquisition, (ii) data processing and (iii) datainterpretation. The success of the prospecting operation generallydepends on satisfactory completion of (i), (ii) and (iii). For example,a seismic source is used to generate an acoustic signal that propagatesinto the earth and that is at least partially reflected by subsurfaceseismic reflectors. The reflected signals are then detected and recordedby an array of seismic receivers located at or near the surface of theearth, in an overlying body of water, or at known depths of boreholes.

FIG. 1, for example, shows a typical setup for offshore seismic surveys,where a large seismic survey vessel 10 is used to tow acoustic receivers12, also known as streamers, suspended below the surface, which carryhydrophones (not shown). During data acquisition, sound waves 14 aretransmitted from the vessel 10 using compressed air guns 16 and whichtravel down through the seabed and reflect back from the differentlayers (strata) of rock 18, 20, 22. The reflected sound waves 14 arethen received by the hydrophones (not shown) located along the seismicstreamers which, when processed, can be used to provide a visualrepresentation (2D, 3D) of the substrata.

Typical seismic traces of the reflections (amplitudes) are shown in FIG.2. The data shown in FIG. 2 has been migrated (“poststack”), i.e. theamplitudes of the traces are moved to their true subsurface position(depth) to eliminate any offset between the source and receiver. Thus,the poststack seismic reflectivity data comprises of a processedacoustic record of subsurface reflections occurring at many differentpoints within the earth's subsurface. In the field of oil and gasexploration and production, specially trained geoscientists would theninterpret images of this poststack data in order to develop a model ofthe subsurface mapping geological features based on the shape and extentof the reflectors within the dataset.

However, seismic horizon interpretation of 3D seismic data is achallenging and time consuming problem including, for example,associating for each trace the seismic event that belongs to aparticular horizon. A number of automated approaches exist to track, forexample, a horizon based on user selected amplitude and phase. Theseseed-based auto-tracking methods propagate through seismic data byvertically matching trace signature information (i.e. extrema) from oneor more seeds to the extrema of adjacent traces within a limited timewindow.

Different seed-based auto-tracking propagation methods may includeconventional line-by-line trace selection, first-in-first out (FIFO),and best match criteria. In complex data, the existence of faults,doublets or noise, and the method of trace traversal, can generatehorizons that are significantly different. This is because lateralcontinuity is largely ignored, therefore resulting in jumps across phasecycles. Consequently, a number of global approaches have been proposedthat consider the lateral continuity during automated 3D tracking.

For example, Edo Hoekstra in US publication US20110307178A1, “Segmentidentification and classification using horizon structure”, describes aprocess to automatically define horizon patches from seismic and/orattribute data. Each horizon patch is formed by determining theprobability of each point belonging to a defined horizon segment. Theprobability is calculated by carrying out geometrical calculations.

Pauget Fabien et al, in US patent US2011246157A1, “Method forgeologically modelling seismic data by trace correlation”, describes amethod for developing a geological model by sampling seismic data basedon a predetermined gradient and a set of traces, each of which hassample points. By analysing the similarities between the seismic dataaround each sampling point, a series of stacked event patches ofrelative geological age is formed. The connections of each geologicalage patch are assessed against patches in neighbouring stacks. Horizonscan then be extracted from a full geological model.

Yingwei Yu et al, in publication “Automatic Horizon Picking in 3DSeismic Data Using Optical Filters and Minimum Spanning Tree”, describesa pattern recognition-based horizon-picking method, which uses anorientation vector and a minimum spanning tree to gain more lateralcontinuity when performing automated 3D horizon tracking.

Purves et al, in GB patent GB2503506B, “Adaptive Horizon tracking”,describes an automated 3D tracking method that gains lateral continuityknowledge by calculating the accumulated cost of a horizon propagatingthrough a predefined gated region located forward of the current horizonboundary.

It is fully accepted that 3D auto-tracking is a very useful and powerfultool for capturing events from largely unambiguous data. Interpreterscan spend a vast amount of time building up a cognitive understanding ofthe data, while at the same time, validating the results from 3Dauto-tracked horizons to subsequently edit any errors that are caused bythe auto-tracking process.

Though, aforementioned approaches try to avoid some of the pit falls of3D auto-tracking by improving the trace-to-trace lateral awareness andcontinuity, in more challenging data, interpreters often revert back tomore traditional methods of manual or semi-automated 2D tracking. Withthese traditional methods, every ‘n’ inlines and ‘m’ crosslines that areforming an interpretation grid are interpreted separately. The frequencyof inlines and crosslines (i.e. the distance between respective inlinesand crosslines) varies depending on the complexity of the data, and itis not uncommon to interpret every 1 to 25 data slices.

Furthermore, the cells within the interpretation grid are generallyfilled using one of two methods, i.e. (i) simple interpolation or (ii)constrained horizon tracking (within each grid cell). Both methods forma resultant horizon surface. Also, interpolation methods by their verynature do not follow the data, but instead generate new data pointswithin the range of known data points, i.e. the inlines and crosslines.The resulting horizon is accurate at each known data point, but can beinaccurate away from these points. Higher accuracy may be accomplishedby increasing the frequency at which inlines and crosslines are manuallyinterpreted. However, this also significantly increases time and effort.

Furthermore, constrained horizon tracking often uses waveformpropagation to vertically match trace signature information (i.e.extrema) from one or more seeds to the trace signature information(extrema) of adjacent traces, within a limited time window and betweenintersecting inlines and crosslines. Due to the lack of global lateralawareness, extrema points can be linked across seismic phase cyclespotentially causing inaccuracies in the horizon surface. Though, as withthe interpolation method discussed previously, constrained waveformpropagation quality may be improved by increasing the frequency ofinlines and crosslines.

Still, both methods, i.e. (i) simple interpolation and (ii) constrainedhorizon tracking, suffer from 3D editing limitations, because in orderto correct any horizon surface inaccuracies, the interpreter has tofirst manually delete any problem areas or regions.

A number of semi-automated 2D tracking algorithms exist to track eventson inline- or crossline slices. For example, local amplitudes arecorrelated between neighbouring traces, with the algorithm stopping atfaults or other discontinuities.

However, these techniques, to a lesser extent, suffer from the samelateral continuity issues as the previously discussed 3D automatedtracking, i.e. propagating any errors made in complex or noisy data.

In order to address the issues mentioned above, Goldner et al., “2DHorizon Tracking using Dynamic Programming”, use a graph to calculate anoptimal path between two given picks by using a global optimizationfunction to find an event with the highest accumulated similarity value.Accumulated similarity values can be somewhat resistant to noise indata, but may fail to stop at faults, due to lower accumulated valuesfrom smaller event segments. This can be somewhat frustrating forinterpreters who wish to manually QC (quality check) event jumps atfaults.

Furthermore, in complex seismic data, interpreting on 2D planes (eitherinlines or crosslines) can be challenging, because alternate routes canlook equally plausible until they are QC'ed (quality checked) in theother direction.

Accordingly, it is an object of the present invention to provide amethod and system suitable to overcome any of the above mentioneddisadvantages. In particular, it is an object of the present inventionto provide an improved automated tracking method and system that issuperiorly adaptable to the seismic data, the geological environment, aswell as, any user input.

SUMMARY OF THE INVENTION

Preferred embodiment(s) of the invention seek to overcome one or more ofthe above disadvantages of the prior art.

According to a first embodiment of the invention there is provided acomputer-implemented method for detecting at least one natural contourof a geologic object in 3D seismic data, the method comprising the stepsof:

-   (a) receiving at least one first predetermined data set from said 3D    seismic data comprising a plurality of phase events;-   (b) selecting at least one first seed phase event having a first    phase characteristic from said plurality of phase events;-   (c) determine a characterising score between said selected at least    one first seed phase event and each one of a predetermined number of    candidate phase events of said at least one first predetermined data    set;-   (d) assign said characterising score to each one of said    predetermined number of candidate phase events;-   (e) adjust said characterising score of at least one of said    predetermined number of candidate phase events in accordance with at    least one first boundary condition;-   (f) determine at least one natural contour between said at least one    first seed phase event and at least a second phase event, utilising    an optimisation algorithm;-   (g) generate a visual representation of said at least one natural    contour within said at least one first predetermined data set.

The method of the present invention provides the advantage of avoidingthe detrimental issues for lateral continuity in both 2D and 3Dauto-picking while optionally maintaining the natural fault boundaries,consequently, allowing the interpreter to explore alternate routes forany tracked geological object (e.g. horizon, fault) to further improvehis conceptual understanding of the data. Furthermore, the system/methodof the present invention comprises a fully integrated 2D/3Dinterpretation environment allowing the user to pick any phase event ona trace (in either a 2D window or in 3D) to generate both, the routethrough the selected data, but also any alternate routes through theselected data. Therefore, the present invention provides a method/systemfor different interpretation practices such as 2D inline/crosslineinterpretation followed by grid filling, or full 3D auto-tracking with2D inline/crossline QC (quality check) with significantly improvedeffectivity, accuracy and ease of use.

Advantageously, said at least one first data set may comprise any one ofa 2D inline slice data set, a 2D crossline slice data set, or a 3Dvolume data set.

Advantageously, said at least one first boundary condition may compriseat least one geologic constraint and/or at least one stratigraphicconstraint and/or at least one algorithmic optimisation constraint inaccordance with at least one variable of said at least one candidatephase event. Preferably, said at least one variable may be any one orany combination of (i) a relative position of said at least onecandidate phase event with respect to any other one of saidpredetermined number of candidate phase events and/or a predeterminedgeological object, (ii) an angular inclination of said at least onefirst natural contour with respect to said 3D seismic data, (iii) saidcharacterising score of said at least one candidate event, and (iv) acharacterising score projected from at least one second predetermineddata set from said 3D seismic data.

Even more advantageously, said at least one second predetermined dataset may comprise any one of a 2D in-line slice data set, a 2D cross-lineslice data set, or a 3D volume data set that is sequentially arranged torespective said at least one first data set within said 3D seismic data.

Preferably, said at least one algorithmic optimisation constraint maycomprise at least one hard constraint and/or at least one softconstraint.

Advantageously, each one of said predetermined number of candidate phaseevents may be eligible in accordance with at least one algorithmiccondition. Preferably, said algorithmic condition may be a Degree ofFreedom (DOF) for movement from one phase event to another.

Advantageously, said first phase characteristic may be any one of apeak-positive amplitude, a trough-negative amplitude or a zero-crossing.

Advantageously, said at least one natural contour in step (f) mayinclude a first natural contour, that is an optimum solution provided bysaid optimisation algorithm, and at least one alternate natural contour,that is an alternate solution provided by said optimisation algorithm.

Advantageously, said at least one second phase event may be a secondseed phase event selected by the user. Even more advantageously, saidstep (c) may include determining said characterising score between saidselected at least one first and second seed phase event and each one ofsaid predetermined number of candidate phase events of said at least onefirst predetermined data set.

Alternatively, said at least one second phase event may be a candidatephase event determined in accordance with its location and/orcharacterising score within said at least one first predetermined dataset.

Advantageously, step (c) may include utilising characteristicinformation from a predetermined number of phase events proximate torespective each one of said predetermined number of candidate phaseevents.

Advantageously, said characterising score is a similarity score.Preferably, said similarity score may be based on at least one attributederivable from said at least one candidate phase events.

Advantageously, when said at least one first data set comprises any oneof a 2D in-line slice data set or a 2D cross-line slice data set, saidoptimisation algorithm may be a ‘Shortest-Path’ algorithm.

Additionally, when said at least one first data set comprises a 3Dvolume data set, said optimisation algorithm may be a ‘Markov RandomField’ optimisation.

Preferably, said geological object may be any one of at least onehorizon feature and at least one fault feature.

According to a second embodiment of the invention there is provided acomputer system for detecting at least one natural contour of a geologicobject in 3D seismic data by a method according to the first embodimentof the present invention.

According to a third embodiment of the invention there is provided acomputer-readable storage medium having embodied thereon a computerprogram, when executed by a computer processor that is configured toperform the method according to the first embodiment of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:

FIG. 1 shows a typical setup for an offshore seismic survey using anarray of acoustic receivers (i.e. hydrophones) and sound waves generatedby an air gun;

FIG. 2 shows a typical plot of migrated reflection traces recorded bythe acoustic receivers after activating the air gun;

FIG. 3 shows a flowchart illustrating an example workflow of the methodof the present invention;

FIG. 4 shows an illustration of phase events (voxels) and ‘voxel lag’,as well as, scores between a selected seed and eligible phase events(peaks, zero-crossings, troughs);

FIG. 5 shows an illustration of phase events without the zero-crossingphase events, including similarity scores between the seed phase eventand all other eligible phase events, as well as, the freedom of movementbetween the seed phase event and all other eligible phase events(arrows);

FIG. 6 shows another illustration of phase events (withoutzero-crossing), where two seed phase events are selected by the userforming a parallelogram of possible paths between the two seed phaseevents;

FIG. 7 shows an illustration of how the accumulated score is calculated,(a) starting from the seed phase event adding the score of the eligiblephase events, (b) subsequent traversal from the phase event with thehighest accumulated score to the next eligible phase event, and (c)traversing back from any phase event via the optimum path;

FIG. 8 shows an illustration of the path through the phase events withthe best accumulated score;

FIG. 9 shows an illustration of using a probability density function(PDF) to calculate the probability that phase events (voxels) belong toa particular event, wherein a number of phase events from around theseed phase event are used;

FIG. 10 shows an illustration of a path through the phase events withthe best accumulated score (a) before and (b) after score adjustmentshave been applied;

FIG. 11 shows an example 2D trace stack including highlighted faults;

FIG. 12 shows an illustration of multi-dimensional scoring, includingscores from two seed phase events (‘black’, scored ‘1’);

FIG. 13 shows an example 2D trace stack including a tracked horizon(‘white’ line) and projected previews of interactively altered paths(dotted line);

FIG. 14 shows an example 2D trace stack and a tracked horizon (‘white’line) as well as, selected alternate routes (respective dotted anddashed ‘white’ and ‘black’ lines);

FIG. 15 shows an illustration of a graph theory set-up in 2D data tocalculate the shortest path for a 2D horizon line utilising the ‘minimumcut’;

FIG. 16 shows an illustration of a graph theory set-up in 3D data tocalculate a minimum surface for a 3D horizon (2×2 in size and apotential of 2× elevation values per phase event);

FIG. 17 shows an illustration of an interpreted line and its projectedpath through a ‘new’ seed phase event using a cost function;

FIG. 18 shows an illustration of a projected line from a ‘new’ seedphase event to peak points on the ‘new’ interpreted line;

FIG. 19 shows a projected line from the ‘new’ seed phase event to peakpoints on the ‘new’ interpreted line in both, inline and crosslinedirections;

FIG. 20 shows an example 3D trace stack (a) in its 3D shape with awrapped tracked line (‘white’ line), and (b) when unfolded (‘white’line);

FIG. 21 shows another example 3D trace stack (a) with a wrapped trackedline, (b) the wrapped tracked line extracted, and (c) the surfacegenerated within the wrapped tracked line constraints;

FIG. 22 shows another example 3D trace stack (a) including a wrappedtracked line and projecting surface fill, and (b) the extracted wrappedtracked line with projected surface fill, and

FIG. 23 shows another example of a fault surface tracked and visualisedwithin a 3D seismic data set.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The exemplary embodiments of this invention will be described inrelation to tracking and interpreting horizons in 2D slices, as well as,3D seismic data. However, it should be appreciated that, in general, thesystem and method of this invention will work equally well for trackingand interpreting any other geologic object (e.g. faults).

For purposes of explanation, it should be appreciated that the terms‘determine’, ‘calculate’ and ‘compute’, and variations thereof, as usedherein are used interchangeably and include any type of methodology,process, mathematical operation or technique. The terms ‘generating’ and‘adapting’ are also used interchangeably describing any type of computermodelling technique for visual representation of a subterraneanenvironment from geological survey data, such as 3D seismic data. Inaddition, the terms ‘vertical’ and ‘horizontal’ refer to the angularorientation with respect to the surface of the earth, i.e. a seismicdata volume is orientated such that ‘vertical’ means substantiallyperpendicular to the general orientation of the ground surface of theearth (assuming the surface is substantially flat), and ‘horizontal’means substantially parallel to the general orientation of the groundsurface of the earth. In other words, a seismic data volume is thereforein alignment with respect to the surface of the earth so that the top ofthe seismic volume is towards the surface of the earth and the bottom ofthe seismic volume is towards the centre of the earth. Furthermore, theterms ‘phase events’ and ‘trace extrema’, as well as, ‘voxels’ and‘nodes’ are used interchangeably.

In the preferred embodiment of the present invention, an automatedmethod/system is provided for adaptively and interactively trackinggeologic features, such as, for example, horizons and faults, in 3Dseismic data. Referring now to the flowchart shown in FIG. 3, theinventive method includes the steps of receiving a predetermined dataset from a 3D seismic data 100. The predetermined data set may be 2Dslices (e.g. inlines or crosslines) or a 3D data set, each comprising aplurality of phase events (i.e. characteristic events of a seismictrace). A user (e.g. interpreter) picks at least one characteristicphase event as a seed point anywhere within the predetermined data set,and a similarity score is determined and assigned to any applicable oneof the other phase events within the predetermined data set 102. Anoptimisation algorithm is then applied to determine the likely contourof the horizon (or fault) starting from the seed point to an endpointwithin the predetermined data set. The end point may be a phase event,having phase characteristics and/or a position within the predetermineddata set, suitable to provide an optimised path (e.g. optimised costfunction) through the predetermined data set, or it may be a second seedpoint picked by the user. The generated optimum path of the trackedgeological feature (e.g. horizon) is that visually presented within thepredetermined data set. Optionally, visual representations of alternate(but less optimal) paths may be provided.

Detailed information on each of the method steps and data processingsteps, as well as, suitable algorithms used, is provided in thefollowing sections.

(i) 2D Inline/Crossline Interpretation A shortest path algorithm may beused to find an optimum path between two points on a seismic slice. Thealgorithm includes determining a score (such as similarity) for eachreachable voxel (i.e. eligible phase event within the predetermined dataset) between the points (i.e. selected seed point and any of the otherphase events), and determining a cost of traversing the voxels.

Basic Shortest Path Algorithm

Starting with a user-defined pick (representing an event peak, trough orzero crossing) on an inline or crossline slice, a score between theuser-defined seed and every reachable voxel (voxels/events within thedata slice that may be ‘reached’ from the outgoing seed towards adestination based on initial algorithm constraints and/or a maximumvertical jump) is calculated. From a single seed, the destination pointcan be ‘open-ended’ (i.e. it is not fixed). This means that thedestination point (chosen by the system) may be the end point having thebest score in the final trace (at the extent of the slice). For example,as shown in FIG. 4, a seed may be picked on a peak event and similarityscores are determined within a two-voxel vertical lag.

The score used in this example is a measure of similarity between theseed point and the other eligible voxels. However, it is understood bythe person skilled in the art that other suitable phase eventcharacteristics may be used to provide a score. The score is then usedto calculate a cost of traversing between the voxels (e.g. from thepicked seed point to the end point). The preferred maximum vertical lagbetween traces is five voxels, although, there is no intrinsic limit. Inthis particular example, the depicted path traversal is limited to a lagof one voxel and does not display the zero-crossings, so as to simplifyillustration as shown in FIG. 5.

When constructing the accumulated cost/score graph, various alternatepaths are evaluated, wherein traversal nodes are restricted to onlythose that can (a) be reached from the starting seed point and (b) reachthe end point (e.g. second seed), so that a path between the startingseed point and the end point is guaranteed.

For example, when using only one seed point, the graph forms a conemoving outward from that starting seed point. However, when an end pointis picked, the forward moving cone is essentially intersected by a conemoving from the picked end point in the opposite direction, resulting ina parallelogram of possible paths. FIG. 6 illustrates such a scenario.

Furthermore, and as shown in FIGS. 7 (a), (b) and (c), alternate pathsthrough the data are determined by calculating an accumulated score andrecording the preferred routes. Starting from the seed point (e.g. nodeA in FIG. 5(a)), the score of the seed point is added to the score ofall its traversal accessible voxels (i.e. nodes B1, B2, and B3 in FIG.5(a)) and the parent score is recorded. Subsequent traversal points(e.g. C1, C2, C3) record the parent score from the node with the highestaccumulated score (e.g. B2), which in turn gets added to its own score(C1, C2, and C3 in FIG. 5(b)).

Because the parent score is the score recorded for the incoming nodewith the best accumulated score, by the time the traversal reaches theend point, the optimum path is already constructed. This concept isillustrated in FIG. 5(c), where the return path is plotted from any nodeback towards the original seed point.

In the case that the end point is not fixed (i.e. picked by the user),the algorithm is adapted to scan along the final trace (i.e. the lasttrace within the predetermined data set) and determine the most suitableend point (i.e. the end point with the best accumulated score). Anexample with a non-fixed end point is illustrated in FIG. 8. Here, thebottom node of the last trace has the largest score (i.e. ‘0.98’) withinthat trace. The route is plotted from that node to each respectiveparent.

It is understood that finding a shortest path in a graph representationof visual data, such as illustrated in FIG. 8, is a well-known approach.However, in order to prevent geologically unfeasible tracking of eventsin the inherently complex and variable seismic data, additionalconstraints have to be provided within the tracking method. Theconstraints may be, for example, angular constraints when trackingthrough the phase events.

Though, instead of using intrinsic constraints, the present inventioninstils additional layers of geological knowledge into the scoringprocess, adapting to the additional information as it becomes available.Such a cybernetic loop ensures that an interpreter's knowledge andexpertise is not only captured but utilised, while the interpreter'sconceptual model of the presented geology increases. In addition, theadaptive nature of the present invention provides a tracking method thatis capable to testing the interpreter's conceptual model by generatingalternative solutions that are visualised as proposed alternatives.

The score at each voxel location (i.e. phase event) may be generatedutilising the following components:

Similarity:

The initial score component that measures similarity between the userspecified seed(s) and all other ‘reachable’ voxels (eligible phaseevents).

Geological Behaviour:

The desired behaviour of a tracked event can be encoded into the scoreby adjusting the score to reward established geological behaviour anddiscourage infeasible geological behaviour.

Bifurcation QC:

If the user wishes to verify or control the choices of a path thoughfaulted regions, the score at these regions can either highlightdecision points or be used to stop at decision points.

Projected Score Enhancement:

Here, the method utilises ‘knowledge’ (i.e. characteristic information)from previous interpretation(s) (e.g. a preceding 2D slice) to‘influence’ the score of the new interpretation.

Multi-Dimensional Score:

When the user/interpreter, for example, adjusts a tracked event path, oradds a new seed, information relating to how the trace signature changesalong that event is captured, creating a multi-dimensional score.

Score: Similarity Measures

Preferably, an n-dimensional probability density function (PDF) is usedto calculate the probability that voxels belong to a particular event(e.g. one-dimension to represent a seismic volume or attribute, orthree-dimensions to represent three frequency magnitude volumesdisplayed with an RGB colour blend). Advantageously, a PDF is generatedfrom a plurality of traces around the user picked seed point, preferablycontaining information from one or more attribute volumes (e.g. seismic,attribute and frequency blend data).

The use of data from neighbouring phase events improves the stability ofthe results, as well as, reducing the quality dependency on theoriginally picked seed. For that, and as illustrated in FIG. 9, a simpleregion growing algorithm may be used to extract, for example, the tenvoxels that have the closest amplitude similarity to the picked seedpoint. The score, which is a probability, is normalised.

Other similarity measures that do not use a PDF may include amplitude orwaveform correlation based on seismic data, where the resultantcorrelation is normalised.

Score: Geological Behaviour

To prevent tracking of non-geological horizons, the score may bemodified to encourage tracked paths that follow data with the same signas the originally picked seed point and to discourage phase jumps (e.g.jumps from peak to trough). This may be provided by applying a sigmodalfunction to the similarity calculation, therefore enhancing the score ofnodes with a strong similarity and reducing the score of nodes with weaksimilarity.

Also, reducing the score of nodes with weak similarity helps todiscourage non-geological tracking behaviour, such as traversing througha phase change event in order to achieve a higher accumulated score (seefor example FIG. 8).

Furthermore, in the preferred embodiment of the present invention, thescore is not reduced according to vertical path steepness, although, apreferred limit may optionally be applied so as to discourage steeppaths.

An example of discouraging non-geological tracking is illustrated inFIGS. 10 (a) and (b), where a phase jump tracked in FIG. 10(a) has beenprevented by adjusting the score changing the tracked optimum path asshown in FIG. 10(b).

Score: Bifurcation QC (Quality Check)

One of the advantages of using score based minimum cost algorithms forhorizon tracking is a significantly improved lateral continuity byallowing tracking to bypass natural boundaries, such as faults. However,this feature can also become a disadvantage, in particular, when theinterpreter wants a tracked horizon to stop at specific discontinuities.FIG. 11 shows an example of a 2D trace stack including highlighted faultregions.

To encourage a path(s) to stop at faulted regions, penalties may begiven to paths crossing faulted regions. In this particular example, afaulted region may be defined by a sudden change of the score (e.g.periods of low score or high costs) over several neighbouring traces, afault attribute, fault stick or fault surface.

Therefore, encoding additional ‘geologically understanding’ into boththe local and global decision making could help to (i) intelligentlydecide whether a discontinuity continues further on, (ii) determinewhere an event may stop (e.g. where events on-lap, down-lap, etc.),(iii) decide the optimum data to track on either seismic data orfrequency blended data, and (iv) provide a global sense of quality.

Such a ‘geological understanding’ may be encoded using arule-based-system or Deep Learning (DL) and/or Artificial Intelligence(AI) systems. For example, successful tracking behaviour throughfaulted, noisy or chaotic data (or any other data characteristics), ispresented to a Deep Learning (DL) or Artificial Intelligence (AI)system, so as to ‘learn’ the desired behaviour through such data. DeepLearning and Artificial Intelligence systems are well known in the artof computer modelling and other computer implemented applications, andare therefore not discussed in any more detail.

Score: Projected Score Enhancement

It is common practise that interpreters track inlines and crosslines insequence, such that multiple inlines will be first interpreted followedby multiple crosslines (or vice versa). The strata between sequentialinlines and crosslines change gradually, therefore, a horizoninterpreted on one slice will normally not change dramatically on thenext subsequent slice.

In order to benefit from the experience and knowledge already instilledin tracked events of previous slices, the score on the currently trackedslice may be influenced from information available on any previousslice, for example, by ‘boosting’ the score values of all phase eventsat node locations with close proximity of the previously interpretedpath.

A projection from the previously tracked phase event may then bedisplayed on the currently tracked slice together with a suggested newevent path. Modifications to existing tracked events may beautomatically cascaded to subsequently tracked lines. This isparticularly important as the present invention is adapted toautomatically update, for example, a 3D surface when an interpreted lineis altered by the user.

Score: Multi-Dimensional Score

A seismic trace signature that is centred on the phase event of interestmay vary significantly over a slice. Consequently, when a user adjustsan event path or adds a new seed point, the information relating to howthe trace signature changes along the event is captured and the score isupdated. As shown in FIG. 12, a path between two user defined seedpoints may, in effect, be formed from a multi-dimensional score, inaddition to calculating the similarity, adjusting for preferredgeological behaviour etc., from each picked seed point to eachrespective ‘reachable’ voxel. The multi-dimensional score may becombined by either calculating the average between the plurality ofscores for each phase event (voxel) or taking a maximum value from theplurality of scores for each phase event.

In chaotic areas the user/interpreter may simply wish to ‘guide’ thetracked path and not utilise the trace signature information from thatlocation, so a chaos attribute may optionally be used to automaticallyexclude seed points that are located in chaotic areas from themulti-dimensional scoring.

Interactive Path & Score

One of the advantages of the present invention is that a tracked path isguaranteed to intersect any user defined seed point(s), so the user canexplore the effect of altering a route of a tracked path by simplyadding additional seed points. As a result, the user is presented with apreview of a new tracked path through that picked seed point that, inessence, acts like a temporary seed point (for example, the temporaryseed point may be defined at the location of the user's mouse pointer onthe screen, see FIG. 13). The trace signature at the temporary seedpoint location may also be included in the multi-dimensional score.Therefore, when a user moves the mouse pointer to new locations withinthe 2D (or 3D) trace stack, it is possible to make an immediateassessment on how a new path might ‘flow’ through the predetermineddata.

Alternate Route(s)

Referring now to FIG. 14, there may be several routes through the dataset where the final accumulated cost is very similar. These alternateroutes are optionally presentable to the user, who may or may not selectany of the alternate routes.

(ii) 3D Surface Interpretation

An embodiment of the present invention is capable of tracking naturalcontours (i.e. surface) of geological features in a 3D seismic tracedata set. The auto tracking algorithm of the present invention,therefore, overcomes one or more of the disadvantages of the prior art.Some of the disadvantages present in the prior art are summarised asfollows:

-   -   Lateral continuity is largely ignored in first-in-first out        (FIFO), and best match region growing, therefore, resulting in        jumps across phase cycles;    -   Interpolating between inline and crossline grids does not follow        the data and requires more frequent interpolation lines in poor        data;    -   Global approaches are slow to process in 3D data and can still        require significant editing in complex data;    -   Editing 3D tracked surfaces involves deleting at least a section        of that surface;    -   Editing 3D tracked surfaces does not provide a projection of        where a new surface might end up after tracking;    -   Tracked 3D surfaces do not updated automatically when, for        example, adjustments are made in 2D slices.

Compared to the prior art, the 3D surface algorithm of the presentinvention is capable of adapting to additional information madeavailable at any time. I.e. the 3D surface is automatically adapted toany new or edited user defined seed points, or any interpreted 2D lines.

Therefore, the key characterising features of the 3D surface algorithmof the present invention are:

-   -   the tracked surface is guaranteed to intersect with any        interpretation (i.e. single seed points, lines, or bounded        regions);    -   the tracked surface will update itself without prior user        deletion;    -   the proportion of the tracked surface that is automatically        updated is calculated dynamically;    -   the tracked surface does not suffer from lateral continuity        issues;    -   editing any point of the tracked surface will prompt a preview        of the proposed new tracked surface;    -   compared to interpolation, fewer tracked interpretation 2D lines        are required to accurately represent a desired event.

The present invention, therefore, provides a new 3D surface algorithmthat extends a 2D graph cut shortest path algorithm into a 3D data set.

3D Filling

When solving the problem of tracking a 3D surface based on growing thatsurface from one or more seed points (such as with region growing) issubject to the same error propagation concerns that are present in 2Dtracking. The problems related to error propagation may be abated byoptimising the score (or minimising the cost) for a surface yielding aminimal cost path in 2D.

In the prior art, attempts to extend a minimal cost path into 3D spacehave often been based on linearly combining a series of minimal cost 2Dlines. Such prior art approaches include, for example, joining a seriesof shortest paths from intelligent scissors segmentation. However, anysolution based on constructing a surface from a series of 2D lines iscertainly not guaranteed to produce the minimal cost surface.

Grady L, for example, suggested using “Minimum-cost Circulation NetworkFlow” to produce a 3D surface, however, its complexity could proverestrictively slow on bigger seismic datasets where interactive userinput and ‘live’ previews are required.

In another example, Ishikawa, “Exact optimization for Markov randomfields with convex priors”, describes how a graph may be constructed soas to solve the optimisation of a subset of ‘Markov Random Field’ (MRF)problems (those with a convex prior function). The approach involvescharacterising the problem as a ‘maximum-flow/minimum-cut’ problem,using a graph where the nodes represent the solution/label of eachvariable and the edges represent variable values (e.g. the convex priorthat is evaluated between variables), and where constraints are imposedon potential solutions. Also, each lateral point in a surface may beregarded as a variable and the solution for each variable may be anelevation value.

The (directed) graph is then used to calculate the maximum flow from asource node to a sink node with each edge of the graph having anassigned capacity. Once the maximum flow has been calculated, theminimal cut corresponds to the set of saturated edges in the graph.

In the present invention, the assumption is made that a horizon surfacehas (at most) only one elevation value at any lateral point, so that thesurface itself may be regarded as a MRF with each sampled pointconnected (laterally) to its neighbours. Such a representation of ahorizon surface may then be leveraged to maximise a similarity score (orminimise a cost) associated with an elevation value at each samplepoint. As a result of simplifying the problem, the computationalcomplexity is reduced subsequently increasing the performance,therefore, allowing for near real-time surface visualisation.

3D Surface—2D Component

FIG. 15 illustrates a graph set-up for calculating the shortest path fora 2D horizon line by calculating the minimal cut. In particular, thegraph shows the minimal cut line separating the source node (s) from thesink node (t). Columns of nodes represent each lateral location of ahorizon surface line. Vertical edges between rows of nodes represent theelevation values of each sample point. The graph, therefore, depicts ahorizon line of five points, each of which has one of three elevationvalues associated with it. Indexed from an elevation of ‘0’, the cut ofthe graph shows the horizon line to have the elevation values‘1’-‘2’-‘1’-‘0’-‘2’. The edges in the graph are set to achieve severalaims:

-   -   First edges (continuous ‘grey’ ‘down’ arrows) are present to        ensure that each column is cut only once. By setting first edges        to an infinite capacity, they never become saturated and the        minimum cut can therefore never cut a first edge (note that cuts        are directed from source (s) to sink (t)).    -   Second edges (dotted ‘black’ ‘up’ arrows) and third edges        (‘dashed’ ‘grey’ ‘side’ arrows) have a capacity set according to        horizon similarity scores as calculated for the 2D tracking.    -   Second edges are assigned a capacity value proportional to the        similarity score of the voxel calculated at the corresponding        elevation and lateral location of the edge.    -   Third edges are assigned a capacity value based on the        cost/score determined for travelling between the adjacent        lateral locations at equal elevation. As vertical jumps in        elevation between lateral locations in the surface will        correspond with the cutting of multiple third edges, the cost of        a vertical jump may be regarded as an accumulation of one or        more of the horizontal costs.    -   Fourth edges (continuous ‘black’ arrows from sink (s)) are        present only to facilitate flow through the graph and have        infinite capacity.

3D Surface—3D Component

The formulation of the minimal cost horizon surface as a minimum cutgraph problem can be trivially extended into 3D. Further layers of nodesmay be added to the graph (i.e. for each slice of a volume through whicha horizon surface is tracked) and additional edges may be added betweenadjacent nodes of adjacent layers. FIG. 16 illustrates such an extensioninto the third dimension for a horizon with 2×2 lateral sample pointsand with two possible elevation values per sample point.

3D Surface—Preserving Interpretation Points

As with 2D tracking, it is desirable to preserve any sample points (seedphase event(s)) in the horizon surface that have been picked by theinterpreter/user. In a preferred embodiment of the invention, anyhorizon elevation value may be fixed when solving the minimum cutproblem, by setting the second edge score of the corresponding locationto ‘0’ (so that it is always saturated) and all other second edges ofthe column to an infinite capacity. Such points are guaranteed to be inthe solution and will also be factored into the optimisation.

3D Surface—Dynamic Editing

When editing a 3D horizon surface, three steps are generally involved.Firstly, the section of the surface that would be affected by the editsthe user wants to make is deleted. Secondly, inline/crossline 2Dinterpretation lines (or adding additional seeds) are edited. Thirdly,apply auto-tracking/interpolation to grow/fill the deleted surfacesection.

The present invention allows the user to edit ‘his own’ inline/crosslineinterpretation, while the surface that passes through theirinterpretation is dynamically updated, so prior surface deletion is notrequired. Furthermore, the dynamic surface edit of the present inventionguarantees that a new surface includes newly edited interpretation lines(see subsection ‘Preserving Interpretation Points’), and calculates theextent of the surface that should update.

In addition, the present invention provides a new “line-of-sight” methodto determine, how much surface requires updating based on interpretationedits. For example, if an inline/crossline has been edited (see FIG. 17)so that a new point has been placed on a different event, a line isprojected from the dragged point's new location, stopping at thefurthest visible point on the line, as seen by the dragged point's newlocation. The peak points then determine the extent of the surface whichrequires updating. The surface outside of the peak points are notaffected by the user interpretation edits. FIG. 18 shows a projectedline from the new seed point to peak points on the new interpreted line.FIG. 19 shows a projected line from the new seed point to peak points onthe new interpreted line in both, inline and crossline, directions.

3D Surface—Wrapped Tracking & Filling

The present invention provides a method of wrapped 2D shortest pathtracking that allows a user to pick a seed point on either a 3D probe oran unwrapped slice view of the 3D probe, i.e. a slice showing the probestwo inline and two crossline sides (see FIGS. 20(a) and (b)).

The wrapped 2D shortest path tracking guarantees that the tracked linestarts and ends at the initial seed point and that the 3D probe cornerpoints (unwrapped slice corner points) match. All of the interactivefeatures described for the 2D Inline/Crossline Interpretation also applyto the wrapped tracking. In particular, the wrapped tracking may be usedto constrain the surface fill for one or more regions, either usingtraditional horizon region growing, interpolation or the 3D surfacetracking algorithm of the present invention. To summarise:

Wrapped Tracking—Horizon Region Growing:

The voxels of the wrapped tracked line become seed points for a regiongrowing algorithm, generating a surface within the tracked lineperimeter.

Wrapped Tracking—Interpolation:

A surface interpolation algorithm is performed between the wrappedtracked line's perimeter.

Wrapped Tracking—Proposed Surface Tracking:

The tracked wrapped line forms both a constraint for the surfacetracking algorithm, as well as, guaranteed points of intersection withthe surface.

As shown in FIGS. 21(a), (b) and (c) and FIGS. 22(a) and (b), aninterpretation grid may be generated by wrapped tracking on a series ofjoined or intersecting 3D probes/unfolded probes. Also, in addition towrapped tracking line constraints, all of the above surface fillingtechniques may be used with grids defined by intersecting inline andcrossline slice interpretations, which may or may not intersect withwrapped tracked lines (see FIGS. 22(a) and (b)).

(iii) Fault Tracking

It is understood by the person skilled in the art that the presentinvention may be used for tracking any geological feature, such as, forexample, horizons and faults.

Seismic faults may help trapping hydrocarbons, but they may also causecomplications during field production due to fragmented reservoirs.Consequently, interpreters attempt to understand migration pathways fromthe source to the reservoir by mapping faults and fault networks.Therefore, as is with horizon tracking, fault interpretation remains aconsuming aspect of 3D seismic interpretation, where faults are oftenpicked manually as discontinuities in seismic amplitude or faultattributes data (such as coherency) volumes. Also, the effectiveness ofautomatic fault detection greatly depends on the type of seismic data,i.e. automated fault interpretations applied to complex or poor qualitydata may require significant user editing, resulting in a similar effortand time cost to common manual methods. Furthermore, semi-automatedmethods, i.e. where the interpreter guides the fault extraction, aregenerally more reliable on seismic data with either high complexity orlower quality.

It is therefore understood by the person skilled in the art, that thepresent invention is also suitable for adaptively tracking faults within2D seismic slices, as well as, 3D seismic data volumes. The specificmethod and system will be in accordance with the embodiment describedfor tracking horizons.

It will be appreciated by persons skilled in the art that the aboveembodiments have been described by way of example only and not in anylimitative sense, and that various alterations and modifications arepossible without departing from the scope of the invention as defined bythe appended claims.

1. A computer-implemented method for detecting at least one naturalcontour of a geologic object in 3D seismic data, the method comprisingthe steps of: (a) receiving at least one first predetermined data setfrom said 3D seismic data comprising a plurality of phase events; (b)selecting at least one first seed phase event having a first phasecharacteristic from said plurality of phase events; (c) determining acharacterising score between said selected at least one first seed phaseevent and each one of a predetermined number of candidate phase eventsof said at least one first predetermined data set; (d) assigning saidcharacterising score to each one of said predetermined number ofcandidate phase events; (e) adjusting said characterising score of atleast one of said predetermined number of candidate phase events inaccordance with at least one first boundary condition; (f) determiningat least one natural contour between said at least one first seed phaseevent and at least a second phase event, utilising an optimisationalgorithm; and (g) generating a visual representation of said at leastone natural contour within said at least one first predetermined dataset; wherein, when said at least one first predetermined data setcomprises a 3D volume data set, said optimisation algorithm is a MarkovRandom Field optimisation.
 2. (canceled)
 3. A method according to claim1, wherein said at least one first boundary condition comprises at leastone geologic constraint and/or at least one stratigraphic constraintand/or at least one algorithmic optimisation constraint in accordancewith at least one variable of said at least one candidate phase event.4. A method according to claim 3, wherein said at least one variable isany one or any combination of (i) a relative position of said at leastone candidate phase event with respect to any other one of saidpredetermined number of candidate phase events and/or a predeterminedgeological object, (ii) an angular inclination of said at least onefirst natural contour with respect to said 3D seismic data, (iii) saidcharacterising score of said at least one candidate event, and (iv) acharacterising score projected from at least one second predetermineddata set from said 3D seismic data.
 5. A method according to claim 4,wherein said at least one second predetermined data set comprises anyone of a 2D in-line slice data set, a 2D cross-line slice data set, or a3D volume data set that is sequentially arranged to respective said atleast one first data set within said 3D seismic data.
 6. A methodaccording to claim 3, wherein said at least one algorithmic optimisationconstraint comprises at least one hard constraint and/or at least onesoft constraint.
 7. A method according to claim 1, wherein each one ofsaid predetermined number of candidate phase events is eligible inaccordance with at least one algorithmic condition.
 8. A methodaccording to claim 7, wherein said algorithmic condition is a Degree ofFreedom (DOF) for movement from one phase event to another.
 9. A methodaccording to claim 1, wherein said first phase characteristic is any oneof a peak-positive amplitude, a trough-negative amplitude or azero-crossing.
 10. A method according to claim 1, wherein said at leastone natural contour in step (f) includes a first natural contour, thatis an optimum solution provided by said optimisation algorithm, and atleast one alternate natural contour, that is an alternate solutionprovided by said optimisation algorithm.
 11. A method according to claim1, wherein said at least one second phase event is a second seed phaseevent selected by the user.
 12. A method according to claim 11, whereinsaid step (c) includes determining said characterising score betweensaid selected at least one first and second seed phase event and eachone of said predetermined number of candidate phase events of said atleast one first predetermined data set.
 13. A method according to claim1, wherein said at least one second phase event is a candidate phaseevent determined in accordance with its location and/or characterisingscore within said at least one first predetermined data set.
 14. Amethod according to claim 1, wherein step (c) includes utilisingcharacteristic information from a predetermined number of phase eventsproximate to respective each one of said predetermined number ofcandidate phase events.
 15. A method according to claim 1, wherein saidcharacterising score is a similarity score.
 16. A method according toclaim 15, wherein said similarity score is based on at least oneattribute derivable from said at least one candidate phase events. 17.(canceled)
 18. (canceled)
 19. A method according to claim 1, whereinsaid geological object is any one of at least one horizon feature and atleast one fault feature.
 20. A computer system for detecting at leastone natural contour of a geologic object in 3D seismic data by a methodaccording to claim
 1. 21. A computer-readable storage medium havingembodied thereon a computer program, when executed by a computerprocessor that is configured to perform the method of claim 1.